Electrochemical cells are commonly used in a fuel cell configuration to produce electrical energy from reducing and oxidant fluids, or in an electrolysis cell configuration to produce product gases from a supply fluid such as producing hydrogen and oxygen gas from water. Typical applications employ a plurality of planar cells arranged in a stack surrounded by an electrically insulating frame that defines manifolds for directing flow of reactant and product fluids. Electrochemical cells generally include an anode electrode and a cathode electrode separated by an electrolyte. Enhancing uniformity of distribution and rates of transport of reducing and product fluids from surfaces of the electrodes and throughout the cells increases operating efficiencies of electrochemical cells.
For example, a well-known use of such electrochemical cells is in a stack for a fuel cell that uses a proton exchange membrane (hereafter "PEM") as the electrolyte. In such a cell, a reactant or reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. The hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. The PEM electrolyte consists of a solid polymer well-known in the art. Another common electrolyte used in fuel cells includes phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention. As is well-known however, PEM cells have significant limitations especially related to liquid water transport to, through and away from the PEM, and related to simultaneous transport of gaseous reducing and oxidant fluids to and from the electrodes adjacent opposed surfaces of the PEM. The prior art includes many efforts to minimize the effect of those limitations.
In operation of a fuel cell employing a PEM, the membrane is saturated with water, and the anode electrode adjacent the membrane must remain wet. As hydrogen ions produced at the anode electrode transfer through the electrolyte, they drag water molecules in the form of hydronium ions with them from the anode to the cathode. Water also transfers back to the anode from the cathode by osmosis. Product water formed at the cathode electrode is removed by evaporation or entrainment into a circulating gaseous stream of oxidant, or by capillary action into and through a porous fluid transport layer adjacent the cathode. Porous water transport plates supply liquid water from a supply of coolant water to the anode electrode and remove water from the cathode electrode returning it back to the coolant water supply, as well as serving to remove heat from the electrolyte and electrodes. It is critical that a proper water balance be maintained between a rate at which water is produced at the cathode electrode and rates at which water is removed from the cathode and at which liquid water is supplied to the anode electrode. An operational limit on performance of such a PEM fuel cell is defined by an ability of the cell to maintain the water balance as electrical current drawn from the cell into the external load circuit increases. If insufficient water is returned to the anode electrode, adjacent portions of the PEM electrolyte dry out thereby decreasing the rate at which hydrogen ions may be transferred through the PEM and also resulting in cross-over of the reactant fluid leading to local over heating. Similarly, if insufficient water is removed from the cathode, the cathode electrode may become flooded effectively limiting oxidant supply to the cathode and hence decreasing current flow.
Fuel cell development has endeavored to enhance fluid transport throughout a cell and to thereby decrease weight and cost requirements to meet specific operating demands. For example, in a typical fuel cell the anode and cathode electrodes comprise thin, porous catalyst layers supported by porous support layers in intimate contact with opposed major surfaces of an electrolyte such as a PEM. Water, reducing and oxidant fluids move to, through and away from the catalyst layers through the pores of the support layers. In order to prevent liquid water from blocking movement of gaseous fluids through the support layer pores, it is known to treat the support layer with hydrophobic substances such as hydrophobic polymers. Such a hydrophobic support layer facilitates transport of gaseous oxidizing, reducing and product fluids, while water moves through the support layer as vapor or "percolates" through the layer as a liquid. Additionally, to minimize excess accumulation of liquid water at the cathode thereby restricting access of the gaseous oxidant to the cathode electrode, it is also known to use porous, carbonized, wetproofed sheets adjacent the cathode, as shown in U.S. Pat. No. 4,826,742 to Reiser that issued on May 2, 1989 and is assigned to the assignee of the invention disclosed herein. Further fuel cell development includes use of hydrophobic substances integrated within a catalyst layer on a porous support layer in an alkaline electrolyte fuel cell to establish a network of hydrophobic gas passages communicating with the catalyst particles making up the electrode and simultaneous use of hydrophilic catalytically inactive particles within the same catalyst layer to form liquid transport pathways, as shown in U.S. Pat. No. 5,480,735 to Landsman et al. that issued on Jan. 2, 1996 and is assigned to the assignee of the present invention. Improvements in liquid electrolyte fuel cells also include disclosure of a plurality of impregnations of hydrophobic materials in hydrophilic porous support layers adjacent electrode surfaces shown in U.S. Pat. No. 4,038,463 to Lamarine et al. that issued on Jul. 26, 1977, and creation of hydrophilic electrolyte reservoirs uniformly distributed throughout a surface of hydrophilic porous support layers by partially masking the support layer prior to application of a hydrophobic polymer and thereafter removing the masking material, as shown in U.S. Pat. No. 3,905,832 to Trocciola, both of which patents are assigned to the assignee of the present invention.
It is also known to add porous water transport plates adjacent the support layers to facilitate liquid water transport and cooling throughout the cell; to integrate a humidifying component to add moisture to the gaseous reducing or oxidant fluids entering the cell to limit a possibility of drying out of the electrodes and an adjacent PEM; to integrate a condensing loop external to the cell to condense moisture within an exiting oxidant stream such as by a heat exchange relationship with ambient air and to then return the condensed moisture to the porous support layers adjacent the anode electrode; to render a portion of a phosphoric acid electrolyte electrochemically inactive in a phosphoric acid cell and thereby form a condensation zone adjacent an oxidant gas outlet which zone operates at a cooler temperature than the active portions of the electrolyte to thereby limit electrolyte loss (as shown in U.S. Pat. No. 4,345,008 to Breault and assigned to the assignee of the present invention); and to generate a pressure differential on the anode side of the cell wherein the reacting gases are maintained at a slightly higher pressure than coolant water and anode supply water passing through porous support layers adjacent reducing gas distribution channels so that the pressure differential assists water transport through the porous support layers and cell.
These improvements have significantly enhanced fuel cell operating efficiencies. However, PEM fuel cells in particular still suffer operational limits related to fluid transport capacities of specific cell components. To restrict liquid water accumulation within pores of porous support layers, the layers are often uniformly treated with hydrophobic substances or "wetproofed", thereby restraining liquid water transport through the pores of the wetproofed support layers. Maximum operational capacity of a fuel cell however should provide a porous support layer that enables some liquid water transport through the support layer simultaneous with transport of gaseous materials through the same layer. Accordingly, there is a need for a porous support layer for use in an electrochemical cell such as a PEM fuel cell that enhances gaseous and liquid fluid transport through the layer without permitting flooding of the layer with liquid water.